Catalysts methods for producing acetic acid from ethane oxidation using MO, V, PD and NB based catalysts, processes of making same and methods of using same

ABSTRACT

An oxide catalyst comprising the elements Mo, V, Nb and Pd. The novel catalytic system provides both higher selectivity and yield of acetic acid in the low temperature one step vapor phase direct oxidation of ethane with molecular oxygen containing gas without production of side products such as ethylene and CO. The feed gas is contacted with a catalyst containing Mo, V, Nb, Pd and O in the ratio a:b:c:d:x where a is 1 to 5; b is 0 to 0.5; c is 0.01 to 0.5; d is 0 0.2 and x is a number determined by the valence of the other elements in the catalyst.

This application is a division of application Ser. No. 08/997,913, filedDec. 24, 1997, now U.S. Pat. No. 6,030,920 which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to improved catalysts, methods of making the sameand methods of using the same in a process for low temperatureoxydehydrogenation of ethane to acetic acid without or having reducedproduction of ethylene and CO by-products in the product stream based onthe composition of the catalyst. The invention also relates to a onestep vapor phase catalytic process using the novel catalyst featuringincreased ethane conversion and higher selectivity to acetic acid up to80% at particular process conditions.

2. Description of Related Art

Several publications are referenced in this application. Thesereferences describe the state of the art to which this inventionpertains, and are hereby incorporated by reference.

Utilization of lower alkane (C₁-C₄) as feed stock to produce added valuepetrochemicals is an industrially desired process. Lower alkanes are lowcost and environmentally acceptable because of their low chemicalreactivity. There are only few commercially available chemical catalyticprocesses which utilize lower alkane as a feed, such as butane to maleicanhydride.

Acetic acid is conventionally produced by methanol carbonylation usingexpensive rhodium catalysts in a liquid phase homogeneous reaction. Thisrequires complicated procedures for recovery of the catalyst andisolation of products. More recently, acetic acid has been produced froman expensive raw material, ethylene, with the production of acetaldehydeas a by-product.

The use of molybdenum and vanadium-containing catalyst systems for lowtemperature oxydehydrogenation of ethane to ethylene was reported by E.M. Thorsteinson et. al., Journal of Catalysts, vol. 52, pp. 116-132(1978). This paper discloses mixed oxide catalysts containing molybdenumand vanadium together with another transition metal oxide, such as Ti,Cr, Mn, Fe, Co, Ni, Nb, Ta, or Ce. The catalysts are active attemperatures as low as 200° C. for the oxydehydrogenation of ethane toethylene. Some acetic acid is produced as a by-product.

Several U.S. Pat. Nos. (4,250,346, 4,524,236, 4,568,790, 4,596,787 and4,899,003) have been granted on low temperature oxydehydrogenation ofethane to ethylene. U.S. Pat. No. 4,250,346 discloses the use ofcatalysts of the formula Mo_(h)V_(i)Nb_(j)A_(k) in which A is Ce, K, P,Ni, and/or U, h is 16, i is 1 to 8, j is 0.2 to 10, and k is 0.1 to 5.U.S. Pat. No. 4,454,236 is directed to the use of a calcined catalyst ofthe formula Mo_(a)V_(b)Nb_(c)Sb_(d)X_(e).

The above cited patents make reference to other patents concerned withthe production of ethylene from ethane by the oxydehydrogenation processand all make reference to the formation of acetic acid as a by-product.

U.S. Pat. Nos. 4,339,355 and 4,148,757 disclose oxide catalystscontaining Mo, Nb, V and a fourth metal selected from Co, Cr, Fe, In, Mnor Y for the oxidation/ammoxidation of unsaturated aliphatic aldehyde tocorresponding saturated aliphatic carboxylic acids.

European Patent Publication EP 02 94 845 discloses a process for thehigher selective production of acetic acid by the oxidation of ethanewith oxygen in contact with a mixture of catalysts consisting of (A) acatalyst for oxydehydrogenation of ethane to ethylene and (B) a catalystfor hydration/oxidation of ethylene. The ethane oxydehydrogenationcatalyst is represented by the formula Mo_(x)V_(y)Z_(z), wherein Z canbe one or more of the metals Nb, Sb, Ta, Ca, Sr, Ti and W. The catalystfor hydration/oxidation is selected from a molecular sieve catalyst, apalladium-containing oxide catalyst, tungsten-phosphorus oxides, or atin molybdenum containing oxide catalysts. European Patent PublicationEP 02 94 845 employs a catalyst prepared by a physical mixture of twotypes of catalysts. This patent does not disclose the catalyst of thepresent invention, which is designed in such a way that it has bothoxydehydrogenation and oxygenation properties on the same catalyst.

European Patent Publication EP 04 80 594 is directed to the use of anoxide catalyst composition comprising tungsten, vanadium, rhenium and atleast one of the alkaline metals for the production of ethylene andacetic acid by oxidation of ethane with a molecular oxygen-containinggas. The replacement of tungsten in whole or part by molybdenum carriedout in European Patent Publication EP 04 07 091 results in an increasein selectivity to acetic acid at the expense of the selectivity toethylene.

European Patent Publication EP 05 18 548 is concerned with a process forthe production of acetic acid by ethane oxidation in contact with asolid catalyst having an empirical formula VP_(a)M_(b)O_(x), where M isone or more optional elements selected from Co, Cu, Re, Nb, W and manyother elements, excluding molybdenum, a is 0.5 to 3, b is 0 to 0.1.

European Patent Publication EP 06 27 401 describes the use of aV_(a)Ti_(b)O_(x) catalyst for the oxidation of ethane to acetic acid.The catalyst composition may comprise additional components from a largelist of possible elements. The reference does not disclose any examplesof catalysts comprising those elements in combination with vanadium,titanium and oxygen. Further, recently reported catalysts containingMoVNb promoted with phosphorus can produce a relatively higher yield ofacetic acid as compared to unpromoted catalyst with the production of byproducts such as carbon monoxide, carbon dioxide and ethylene (U.S.application Ser. No. 08/932,075 (Attorney Docket No. 582815-2060), filedSep. 17, 1997, entitled “Catalysts for the Oxidation of Ethane to AceticAcid, Processes of Making Same and Processes of Using the Same”, herebyincorporated by reference). Further, due to environmental lawconstraints, carbon monoxide is a less desirable by-product.

Accordingly, it would be desirable to produce an improved catalyst foruse in the selective production of acetic acid from ethane through asingle stage partial oxidation process without the production of carbonmonoxide and ethylene.

SUMMARY OF THE INVENTION

According to the present invention, ethane is oxidized with molecularoxygen to acetic acid in a gas phase reaction at relatively high levelsof conversion, selectivity and productivity and at temperatures rangingfrom 150° C. to 450° C. and at pressures ranging from 1-50 bar. This isachieved using a catalyst having a calcined composition ofMo_(a)V_(b)Nb_(c)Pd_(d), wherein:

a is 1 to 5;

b is 0 to 0.5;

c is 0.01 to 0.5; and

d is 0 to 0.2.

The numerical values of a, b, c and d represent the relative gram-atomratios of the elements Mo, V, Nb and Pd, respectively, in the catalyst.The elements are present in combination with oxygen in the form ofvarious oxides. The inventive catalysts are preferably produced usingthe methods disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be describedfurther, by way of example, with reference to the accompanying drawings,in which:

FIG. 1 is a graphical representation of an X-ray diffraction (XRD)pattern of a catalyst according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The catalyst is a mixture of the elements in combination with oxygen.The catalyst can also be represented by the formula Mo₁₋₅ V_(0-0.5)Nb_(0.01-0.5) Pd_(0-0.2)O_(y), where y is a number determined by thevalence requirements of the other elements in the catalyst composition.

The catalyst of the invention can be used with or without a support.Suitable supports for the catalyst include alumina, silica, titania,zirconia, zeolites, molecular sieves and other micro/nanoporousmaterials, and mixtures thereof. When used on a support, the supportedcatalyst usually comprises from about 10 to 50% by weight of thecatalyst composition, with the remainder being the support material.

The choice of the compounds used as well as the specific proceduresfollowed in preparing a catalyst can have a significant effect on theperformance of a catalyst. It is believed the elements of the catalystcomposition are in combination with oxygen as oxides.

Preferably, the catalyst is prepared from a solution of solublecompounds (salts, complexes or other compounds) of each of the metals.The solution is preferably an aqueous system having a pH of 1 to 10 andmore preferably at a pH of 1 to 7, at a temperature of from about 30° C.to about 100° C.

Generally, a mixture of compounds containing the elements is prepared bydissolving sufficient quantities of soluble compounds and dispersing theinsoluble compounds so as to provide a desired gram-atom ratios of theelements in the catalyst composition. The catalyst composition is thenprepared by removing the water and/or other solvent from the mixture ofthe compounds in the solution system. The dried catalyst is calcined byheating to a temperature from about 250° C. to about 450° C. in air oroxygen for a period of time from about one hour to about 16 hours toproduce the desired catalyst composition. Generally, the higher thetemperature the shorter the period of time required.

Preferably, the molybdenum is introduced into the solution in the formof ammonium salts such as ammonium paramolybdate, or organic acid saltsof molybdenum such as acetates, oxalates, mandelates, and glycolates.Some other partially water soluble molybdenum compounds which may beused include molybdenum oxides, molybdic acid, and chlorides ofmolybdenum.

Preferably, the vanadium is introduced into the solution in the form ofammonium salts such as ammonium metavanadate and ammonium decavanadate,or organic salts of vanadium such as acetates, oxalates, and tartrates.Partially water soluble vanadium compounds such as vanadium oxides, andsulfates of vanadium can be used. To achieve a complete solubility, acertain amount of oxalic or tartaric acid can be added.

The niobium is preferably used in the form of oxalates or hydrate oxide.Surprisingly, it has been discovered that niobium hydrate oxide is apreferred niobium precursor because it provides significant costadvantages by way of improved niobium yields in the resultant catalyst.According to one preferred embodiment of the invention, niobium hydrateis dissolved in oxalic acid at a temperature of about 85-99° C.,preferably 90-95° C. Preferably, the niobium hydrate oxide:oxalic acidratio is about 1:5. Other sources of this metal in soluble form includecompounds in which the metal is coordinated, bonded or complexed to abeta-diketonate, carboxylic acid, and amine, and alcohol, or analkanolamine.

Preferably, the palladium is introduced into the catalyst slurry in theform of Pd on activated charcoal or alumina or solution of salts ofpalladium such as acetate, chloride, etc.

Preferably, the catalyst is prepared by the following general procedure.Aqueous solution of vanadium, niobium and molybdenum are preparedseparately. The vanadium solution is mixed with niobium solution atparticular temperature and pH. The molybdenum solution is added to a VNbsolution to form a combined gel. The fourth component, palladium, isslowly added to the combined gel solution. After mixing and heating forabout ½ to 2 hours, the resultant gel is dried to incipient wetness withcontinuous stirring at about 100° C.

After drying the resultant gel mixture at 120° C. for 16 hours, thecatalyst is heated to 350° C. at the rate of 2° per minute and calcinedat this temperature in air for 4 hours to produce the desired oxidecomposition. This regime seems to be close to optimum as it allows toobtain a catalyst with the desired structure.

The catalysts disclosed in present invention preferably have a structurewhich is diffused or poorly crystallized patterns and can becharacterized by the X-ray diffraction (XRD) pattern presented in Table1 and FIG. 1.

TABLE 1 Catalyst XRD characteristics Interplanar distance (Å) Intensity4.00 strong 3.57 diffused medium broad 2.01 weak 1.86 weak

A strong reflection at 4.00 Å corresponds to orthorhombic and/orhexagonal MoO₃ phases (Ponselle, L., Wrobel, G., and Germain, J. E., J.Microsc., vol. 7, page 949 (1968)). However, Thorsteinson et. al. hasattributed this peak to Mo-vanadium containing phases in MoVNb catalyticsystem. Generally to obtain this structure, a catalyst has to beprepared by calcining at 350° C. by method described above. The broadpeak at 3.57 is a kind of diffused peak and is difficult to attribute toany one phase especially when sample is calcined at a temperature of350° C. During calcination at 350° C., the partially crystallized phasespecified above is formed which is the active structure in the selectiveoxidation of ethane to acetic acid. Both amorphous or well-crystallinephases obtained by calcination at temperatures lower than 330° C. andhigher than 370° C. are less effective with respect to production ofacetic acid. Further, catalyst disclosed in the present invention doesnot show any additional peaks correspond to Pd containing material, FIG.1.

Another aspect of the invention relates to the production of acetic acidfrom ethane without the production or with significantly reducedproduction of the by-products ethylene and CO in the product stream.

According to one embodiment of the invention, acetic acid is produceddirectly from ethane on a single step vapor phase catalytic processusing the catalyst according to the invention.

The raw material used as the source for the ethane can be a gas streamwhich contains at least five volume percent of ethane. The gas streamcan also contain minor amounts of the C₃-C₄ alkanes and alkenes, lessthan five volume percent of each. The gas stream can also contain majoramounts, more than five volume percent, of nitrogen, carbon dioxide, andwater in the form of steam.

The reaction mixture useful in carrying out the process is generally onemole of ethane, 0.01 to 2.0 moles of molecular oxygen either as pureoxygen or in the form of air, and zero to 4.0 moles of water in the formof steam. The water vapor or steam is used as a reaction diluent and asa heat moderator for the reaction and it also acts as a desorptionaccelerator of the reaction product in the vapor phase oxidationreaction. Other gases may be used as reaction diluents or heatmoderators such as helium, nitrogen, and carbon dioxide.

The gaseous components of the reaction mixture include ethane, oxygenand a diluent, and these components are uniformly admixed prior to beingintroduced into the reaction zone. The components may be preheated,individually or after being admixed, prior to being introduced into thereaction zone.

The reaction zone generally has a pressure of from 1 to 50 bar,preferably from 1 to 30 bar; a temperature of from about 150° C. toabout 450° C., preferably from 200° C. to 300° C.; a contact timebetween the reaction mixture and the catalyst of from about 0.01 secondto 100 seconds, preferably from 0.1 second to 10 seconds; and a spacehourly velocity of from about 50 to about 50,000 h⁻¹, preferably from100 to 10,000 h⁻¹ and most preferably from 200 to 3,000 h⁻¹.

The contact time is defined as the ratio between the apparent volume ofthe catalyst bed and the volume of the gaseous reaction mixture feed tothe catalyst bed under the given reaction conditions in a unit of time.

The space velocity is calculated by determining total reactor outlet gasequivalent in liters of the total effluent evolved over a period of onehour divided by the liters of catalyst in the reactor. This roomtemperature volume is converted to the volume at 0° C. at 1 bar.

The reaction pressure is initially provided by the feed of the gaseousreactant and diluent and after the reaction has commenced, is maintainedby the use of a suitable back-pressure controller placed on the reactoroutlet stream.

The reaction temperature is provided by placing the catalyst bed withina tubular converter having walls placed in a furnace heated to thedesired reaction temperature.

The oxygen concentration in the feed gas mixture can vary widely, from0.1 to 50% or higher of the feed mixture by applying of proper measuresto avoid explosion problems. Air is the preferred source of oxygen inthe feed. The amount of oxygen present may be a stoichiometric amount,or lower, of the hydrocarbons in the feed.

According to one preferred embodiment, the process is carried out in asingle stage with all the oxygen and reactants being supplied as asingle feed with unreacted initial reactants being recycled. However,multiple stage addition of oxygen to the reactor with an intermediatehydrocarbon feed can also be used. This may improve productivity toacetic acid and avoid a potentially hazardous condition.

The catalyst of the invention is not limited to the oxydehydrogenationof ethane to ethylene and acetic acid and may be used for oxidizingalpha-beta unsaturated aliphatic aldehydes in the vapor phase withmolecular oxygen to produce the corresponding alpha-beta unsaturatedcarboxylic acids. Further, the catalyst of the invention can also beapplied for oxidation of ethylene to acetic acid.

Accordingly, another aspect of the invention relates to a process forperforming a catalytic chemical reaction comprising the step ofintroducing one or more reactants into a reaction zone containing thenovel catalyst composition, wherein the catalytic chemical reactionpreferably converts one or more fluid phase reactants to one or morefluid phase products. According to one preferred embodiment, the processoxidizes lower alkenes to produce the corresponding acids (e.g., ethaneto acetic acid, propane to propionic acid, methane to formic acid andbutane to butyric acid).

EXAMPLES

The following examples are illustrative of some of the products andmethods of making the same falling within the scope of the presentinvention. They are, of course, not to be considered in any waylimitative of the invention. Numerous changes and modification can bemade with respect to the invention.

Catalyst Testing

Catalysts evaluation were carried out in a stainless steel fixed bedtubular reactor under standard process conditions. For the ethane-airsystem, the gas feed composition was 15% by volume ethane and 85% byvolume air at a reaction temperature of 260° C., a pressure of 200 psigand at space velocity of about 1,100 h⁻¹ by using 3 g of calcinedcatalyst. For the enriched ethane system (ethane/oxygen), the gas feedcomposition was 82% by volume ethane (enriched hydrocarbon) and 18% byvolume oxygen at a reaction temperature of 250° C., a pressure of 200psig and at space velocity of about 5,000 h⁻¹ by using of 0.3 g ofcalcined catalyst.

Reaction products were analyzed on-line by gas chromatography. Oxygen,nitrogen and carbon monoxide were analyzed using a 2.5 m by 3 mm columnof 13×molecular sieve. Carbon dioxide, ethane and ethylene were analyzedusing a 0.5 m by 3 mm column packed with material sold under the tradename PORAPACK™ N. Acetic acid and water were analyzed using a 1.5 m by 3mm column packed with material sold under the trademark HAYASEP™ Q. Inall cases, the conversion and selectivity calculations were based on thestoichiometry:

The yield of acetic acid was calculated by multiplying the selectivityto acetic acid by ethane conversion.

Three procedures for catalyst preparation were applied for the examplesgiven in Table 1.

Group A: Examples 1, 2, 3, 4, 6, 7, 8, 9 and 12.

Group B: Examples 5, 10, 11, 13, 15 and 16.

Group C: Examples 14, 17, 18 and 19.

TABLE 2 Catalyst Compositions Example No. CATALYST COMPOSITION 1Mo₁V_(0.396)Nb_(0.128) 2 Mo₁V_(0.396)Nb_(0.128)Pd_(4.99E-05) 3Mo₁V_(0.396)Nb_(0.128)Pd_(9.60E-05) 4Mo₁V_(0.396)Nb_(0.128)Pd_(1.44E-04) 5Mo₁V_(0.396)Nb_(0.128)Pd_(1.90E-04) 6Mo₁V_(0.396)Nb_(0.128)Pd_(1.92E-04) 7Mo₁V_(0.396)Nb_(0.128)Pd_(1.92E-04) 8Mo₁V_(0.396)Nb_(0.128)Pd_(1.92E-04) 9Mo₁V_(0.396)Nb_(0.128)Pd_(1.92E-04) 10Mo₁V_(0.396)Nb_(0.128)Pd_(2.68E-04) 11Mo₁V_(0.396)Nb_(0.128)Pd_(2.85E-04) 12Mo₁V_(0.396)Nb_(0.128)Pd_(3.84E-04) 13Mo₁V_(0.396)Nb_(0.128)Pd_(9.99E-03) 14Mo₁V_(0.396)Nb_(0.128)Pd_(1.00E-03) 15Mo₁V_(0.396)Nb_(0.128)Pd_(1.50E-03) 16Mo₁V_(0.396)Nb_(0.128)Pd_(3.00E-03) 17Mo₁V_(0.398)Nb_(0.128)Pd_(3.00E-03) 18Mo₁V_(0.396)Nb_(0.128)Pd_(5.00E-03) 19Mo₁V_(0.396)Nb_(0.128)Pd_(1.00E-02)

Preparation Procedure for Catalyst Group A

Ammonium metavanadate (Aldrich Chemicals, Assay=99.0%) in the amount of11.4 grams was added to 120 ml of distilled water and heated to 90° C.with stirring. 2.5 grams of oxalic acid were added to obtain a clearyellow color solution with a pH between 5 and 6 (Solution A). 19.4 gramsof niobium oxalate (21.5% Nb₂O₅, Niobium Products Company, USA) wereadded to 86 ml of water and heated to 65° C. with continuous stirring togive a clear white color solution with a pH of 1 (Solution B). SolutionB was combined with Solution A. The resultant solution was heated at 90°C. and 28 grams of oxalic acid was added very slowly with continuousstirring to give Solution C.

Ammonium paramolybdate tetra hydrated (Aldrich ChemicalsA.C.S-12054-85-2) in the amount of 43.2 grams was added to 45 ml ofwater and heated to 60° C. to give a colorless solution with a pHbetween 6.0 and 6.5 (Solution D). Solution D was combined slowly withSolution C to give dark blue to dark gray color precipitates (MixtureE). The required amount of palladium was added slowly to gel mixture.This dark color combination was stirred vigorously to achieve ahomogeneous gel mixture which was then dried slowly to incipient drynesswithin 60 to 120 minutes at 95-98° C. with continuous stirring.

The resulting solid was put in a China dish and dried additionally in anoven at 120° C. for sixteen hours. The dried material was cooled to roomtemperature and placed in a furnace. The temperature was raised fromroom temperature to 350° C. at the rate of 2°/min. and thereafter heldat 350° C. for four hours.

Calcined catalyst was formulated into uniform particles of the 40-60mesh size and evaluated for the ethane oxidative dehydrogenationreaction. Examples 7, 8, and 9 were prepared using the same method tocheck the reproducibility of the catalytic materials.

Preparation Procedure for Catalyst Group B

Ammonium metavanadate (Aldrich Chemicals, Assay=99.0%) in the amount of7.6 grams was added to 80 ml of distilled water and heated to 90° C.with stirring. A yellow color solution with pH between 5 and 6 wasobtained (Solution A). 3.4 grams of niobium hydrate oxide (80% Nb₂O₅,Niobium Products Company, USA) and 20 grams of oxalic acid were added to80 ml of water and heated to 95° C. with continuous stirring to give aclear white color solution with a pH of 0.57 (Solution B). Solution Aand B were mixed together at 90° C. with continuous stirring of thecontent of the mixture. Color changes from pale yellow to brown to greento dark green were observed. The pH of the solution was 1.20 at 85° C. 8g of oxalic acid was added very slowly to the solution with continuesstirring of the content of the mixture at 90° C. A dark blue-green colorsolution with a pH of 0.86 at 86° C. was obtained (Solution C).

Ammonium paramolybdate tetra hydrated (Aldrich ChemicalsA.C.S-12054-85-2) in the amount of 28.8 grams was added to 30 ml ofwater and heated to 60° C. to give a colorless solution with a pHbetween 5.0 and 6.0 (Solution D). Solution D was combined slowly withSolution C to give dark blue to dark gray color precipitates (MixtureE). The required amount of palladium as Pd-alumina was-added slowly togel mixture. This dark color combination was stirred vigorously toachieve a homogeneous gel mixture which was then dried slowly toincipient dryness within 60 to 120 minutes at 95-98° C. with continuousstirring.

The resulting solid was put in a China dish and dried additionally in anoven at 120° C. for sixteen hours. The dried material was cooled to roomtemperature and placed in a furnace. The temperature was raised fromroom temperature to 350° C. at the rate of 2°/min. and thereafter heldat 350° C. for four hours.

Calcined catalyst was formulated into uniform particles of the 40-60mesh size and evaluated for the ethane oxidative dehydrogenationreaction.

Preparation Procedure for Catalyst Group C

Ammonium metavanadate (Aldrich Chemicals, Assay=99.0%) in the amount of3.42 grams was added to 90 ml of distilled water and heated to 87° C.(Solution A). 5.82 grams of niobium oxalate (21.5% Nb₂O₅, NiobiumProducts Company, USA) was added to 25.8 ml of water and heated to 63°C. with continuous stirring to give a turbid white color solution(Solution B). Solution B was combined with Solution A. The resultantsolution was heated at 90° C. and 9 grams of oxalic acid was added veryslowly with continuous stirring to give Solution C. Ammoniumparamolybdate tetra hydrated (Aldrich Chemicals A.C.S 12054-85-2) in theamount of 12.96 grams was added to 13.2 ml of water and heated to 50° C.to give a colorless solution (Solution D). Solution D was combinedslowly with Solution C to give dark blue to dark gray color precipitates(Mixture E). The required amount of palladium as Pd charcoal was addedslowly to gel mixture. This dark color combination was stirredvigorously to achieve a homogeneous gel mixture which was then driedslowly to incipient dryness.

The resulting solid was put in a China dish and kept at room temperature(25° C.) for one day. Further catalyst was dried in an oven at 120° C.for sixteen hours. The dried material was cooled to room temperature andplaced in a furnace. The temperature was raised from room temperature to350° C. at the rate of 1° C./min. and thereafter held at 350° C. forfive hours.

The BET surface area of the catalysts mentioned in the examples (1-19)varies from 25 to 35 m²/g.

Two types of feed systems for catalytic evaluation were applied in orderto see the impact of catalyst composition on the product selectivity andactivity.

1) Ethane-oxygen (Ethane enriched)

2) Ethane-Air (Ethane leaned).

The results of the tests with these catalysts under the reactionconditions described above are given in Tables 3 and 4.

TABLE 3 Catalyst Evaluation Data for Ethane-Air System (Reactioncondition: 260° C., 200 Psi, Ethane:Air (15%:85%), F/W 10/min.) YieldSelectivity (%) (%) Exam- Conversion (%) Acetic Acetic ple Ethane oxygenacid CO CO2 Ethylene Acid 1 64.69 100 30.47 22.65 16.08 26.96 19.79 249.08 100 47.75 0 44.8 7.49 23.44 3 48 100 49.69 0 47.42 3.31 23.85 446.76 100 51.52 0 49.3 0 24.09 5 51.03 99.3 48.22 0 51.27 0.88 24.61 647.17 100 52.3 0 48.56 0 24.67 7 49.96 100 54.29 0 43.88 2.34 27.12 849.7 100 53.06 0 43.61 3.85 26.37 9 47.97 100 55.07 0 45.51 0 26.42 1049.19 100 55.74 o 42.71 1.87 27.42 11 49.61 100 60.27 0 38.51 0.55 29.912 49.96 100 53.02 0 47.86 0 24.9 13 44.32 85.57 55.75 1.8 43.69 0.5824.7 14 50.05 100 48.68 0.32 47.61 4.07 24.3 15 48.79 100 48.21 0 51.32.22 23.5 16 44.03 93.55 45.99 0 56.21 0.7 20.2 17 47.02 100 42.61 054.49 3.55 20.0 18 46.9 100 37.42 0 57.3 5.94 17.5 19 47.3 100 48.73 049.52 2.08 23.0

TABLE 4 Catalyst Evaluation Data for Ethane-Oxygen System (Reactioncondition: 260° C., 200 Psi, Ethane:Oxygen (82%:18%), F/W = 10/min.)Yield Selectivity (%) (%) Exam- Conversion (%) Acetic Acetic ple Ethaneoxygen acid CO CO2 Ethylene Acid 1 12.96 47.75 22.46 4.03 2.80 26.962.91 6 8.71 57.26 59.47 1.25 12.82 28.81 5.18 12 9.36 60.86 63.91 0.7614.43 20.63 5.98 14 10.08 70.66 57.54 0.52 15.97 26.30 5.80 17 11.3177.48 67.05 0.4 16.19 15.72 7.59 18 11.51 82.13 65.1 0.2 16.79 15.987.49 19 9.62 79.16 73.71 0.24 21.52 6.03 7.09 1** 20.7 97.67 31.25 6.154.32 60.03 6.47 12** 13.6 100 57.23 0.47 19.24 22.27 7.78 19** 13.57 10066.84 0 25.08 10.45 9.07 **Reaction temperature for these tests is 275°C.

The activities of all the samples were measured using the same amount ofcatalyst (3 g). The differences in ethane conversion might be caused bythe different specific surface areas of catalysts. In this case,catalyst activity can be expressed as the conversion per unit area,Table 5. Evaluation data shows that catalyst without palladium (ExampleNo 1) has a higher specific conversion or activity as compared to Pdfamily catalysts (Pd containing catalyst). Further, activity in terms ofconversion does not change significantly for the palladium familycatalysts regardless of varying amount of palladium in the catalysts.This means that addition of palladium to MoNbV metal oxide decreases thespecific overall activity. However, this change in the activity is notdependent on the amount of palladium.

TABLE 5 Activity of Catalysts BET surace Ethane Conversion SpecificExample area (m2/g) (%) -E-Air system conversions (%/m2) 1 25.6 64.690.8423 2 23.00 48.00 0.6956 4 27.5 46.76 0.5663 6 29.97 47.17 0.5246 726.08 49.96 0.6380 10 28.03 49.19 0.5780 11 29.00 49.61 0.5663 12 28.8149.96 0.5780 *S. Conv is Specific conversion of ethane per unit area =Conversion/specific surface area (%/m2)

Palladium-containing mixed oxide catalysts for ethane-air andethane-oxygen system follows the same selectivity and activity trend,Tables 3 and 4. Total conversion of ethane decreases with the increasein the selectivity of acetic acid. Selectivity to acetic acid passesthrough a maximum with an increase in the amount of palladium in themixed oxide catalysts. Further, the amount of ethylene and carbonmonoxide (primary reaction products) are completely converted to aceticacid and carbon dioxide, depending on the composition of the catalyst.

It is seen that with the addition of Pd to the MoVNb oxide the followingoverall changes in catalyst performance are observed:

1. Rate of oxygenation of ethylene to acetic acid increases and passesthrough maximum with the amount of palladium.

2. Rate of CO oxidation to CO₂ increases. Consequently, a decrease in COselectivity is observed.

The MoVNbPd mixed metal oxide catalysts are redox type catalysts havingan ability to be reduced and reoxidized. Over such type of catalysts,dehydrogenation of alkane is a dominant primary reaction producingdehydrogenated products, alkenes and water at short reaction contacttimes (low hydrocarbon conversion). However, at relatively high contacttimes and conversions, oxygenated and degradation products are formedfrom the secondary reactions producing acids and carbon oxides. At highcontact time there is a competition of total oxidation reactions leadingto CO and CO₂ and oxygenation reactions leading to acids.

Selectivity behavior to desired mild oxidation products depends on thetypes of active centers in the catalysts in addition to other physicalreaction parameters, such as hydrocarbon to oxygen ratio, pressure,temperature and contact time. Further, the interaction of surfaceintermediate with active sites demonstrates the selectivity patterns inoxidation catalysts. Mixed metal oxide phases of MoV are known to beresponsible for the oxidative dehydrogenation of ethane, alkane, toethylene. It has been reported that addition of Nb to MoV oxide improvesthe selectivity to acetic acid (E. M. Thorsteinson et. al.). Furtherpalladium is known as a total oxidation metal as well as helping tofacilitate the oxygenation of alkene. The presently disclosed resultsfor MoVNbPd catalysts demonstrate that there is balance between theoxygenation and total oxidation reaction and this depends on the numberor amount of palladium in the mixed oxide catalyst. At lowconcentrations-of palladium, when Pd atoms are well dispersed at thesurfaces, the oxygenation reaction is favored resulting in highselectivity to acetic acid. While at higher concentrations of palladiumand the formation of metal crystallites is very possible, totaloxidation reaction resulting in higher selectivity to CO₂ is favored.Consequently, decreased in acetic acid selectivity at highconcentrations of Pd in the catalyst is accounted for. Further,catalytic data also demonstrate that the specific overall catalyticactivity of MoVNbPd catalysts having varying amount of palladium doesnot change considerably but the selectivity to acetic acid increasesessentially.

The above description of the invention is intended to be illustrativeand not limiting. Various changes or modifications in the embodimentsdescribed may occur to those skilled in the art. These can be madewithout departing from the spirit or scope of the invention.

What is claimed is:
 1. A single stage catalytic process for directconversion of ethane to acetic acid by means of ethane oxidationcomprising the step of oxidizing ethane in a reaction mixture comprisingethane and oxygen or a compound capable of providing oxygen in areaction zone in the presence of a catalyst composition represented bythe formula: Mo_(a)V_(b)Nb_(c)Pd_(d)O_(x), where a is 1 to 5; b is 0 to0.5; c is 0.01 to 0.5; and d is 0 to 0.2; wherein x is a numberdetermined by the valence requirements of the other elements in thecatalyst composition.
 2. The process of claim 1, wherein said catalystis in the form of a fixed or fluidized bed and said oxidation is carriedout by a feed mixture comprising ethane introduced into the reactionzone.
 3. The process of claim 2, wherein said feed mixture furthercomprises air.
 4. The process of claim 2, wherein said feed mixturecomprises oxygen.
 5. The process of claim 2, wherein said feed mixturecomprises molecular oxygen ranging from 0.1 to 50% by volume of thefeed.
 6. The process of claim 2, wherein said feed mixture is dilutedwith steam in an amount ranging from 0 to 40% by volume.
 7. The processof claim 1, wherein oxidation is achieved while operating in gas phaseat a temperature of from 150 to 450° C., under a pressure of from 1 to50 bars, and with a contact time between reaction mixture and thecatalyst of from 0.1 to 10 seconds.
 8. The process of claim 1, whereinsaid oxidation provides a selectivity to acetic acid of 60% at a 50%conversion of ethane per single pass through said reaction zone.
 9. Asingle stage catalytic process for direct conversion of ethane to aceticacid by means of ethane oxidation comprising the step of oxidizingethane in a reaction mixture feed comprising ethane and oxygen or acompound capable of providing oxygen in a reaction zone in the presenceof a catalyst composition represented by the formula:Mo_(a)V_(b)Nb_(c)Pd_(d)O_(x), where a is 1 to 5; b is 0 to 0.5; c is0.01 to 0.5; and d is 0 to 0.2; wherein x is a number determined by thevalence requirements of the other elements in the catalyst compositionand said oxidation of ethane does not produce CO and ethylene asby-products using molecular oxygen within a range from 0.1 to 50% of thefeed.
 10. The process of claim 1, further comprising the step ofintroducing oxygen into the feed mixture to increase the yield,selectivity or both yield and selectivity of acetic acid.
 11. Theprocess of claim 1, further comprising the step of introducing oxygeninto the reaction zone to increase the yield, selectivity or both yieldand selectivity of acetic acid.
 12. A process for performing a catalyticchemical reaction in fluid phase comprising at least one reactant influid phase under suitable reaction conditions with a catalystcontaining a catalyst composition comprising the elements Mo, V, Nb, andPd in the form of oxides, in the ratio MO_(a)V_(b)Nb_(c)Pd_(d) a is 1 to5; b is 0 to 0.5 c is 0.01 to 0.5; and d is 0 to 0.2.
 13. A process forperforming a catalytic chemical reaction comprising the step ofintroducing a reactant in fluid phase into a reaction zone containing acatalyst composition having a catalyst composition comprising theelements Mo, V, Nb, and Pd in the form of oxides, in the ratioMo_(a)V_(b)Nb_(c)Pd_(d) a is 1 to 5; b is 0 to 0.5 c is 0.01 to 0.5; andd is 0 to 0.2.
 14. The process of claim 12, wherein said catalyticchemical reaction converts one or more fluid phase reactants to one ormore fluid phase products.
 15. The process of claim 12, wherein saidcatalytic chemical reaction oxidizes lower alkenes to correspondingacids.
 16. The process of claim 14, wherein said one or more fluid phasereactants comprise ethane and said one or more fluid phase productcomprise acetic acid.
 17. The process of claim 15, wherein said one ormore fluid phase reactants comprise ethane and said one or more fluidphase products comprise acetic acid.
 18. The process of claim 14,wherein said one or more fluid phase reactants comprise alpha-betaunsaturated aliphatic aldehydes and oxygen and said one or more fluidphase products comprise alpha-beta unsaturated carboxylic acids.
 19. Asingle stage catalytic process for direct conversion of ethane to aceticacid by means of ethane oxidation comprising the step of oxidizingethane in a reaction mixture comprising ethane and oxygen or a compoundcapable of providing oxygen in a reaction zone in the presence of acatalyst made by a process comprising the steps of: (a) combining theelements Mo, V, Nb and Pd in the following ratio to form a compositionhaving the formula: Mo_(a)V_(b)Nb_(c)Pd_(d) a is 1 to 5; b is 0 to 0.5;c is 0.01 to 0.5; and d is 0 to 0.2; and (b) calcining said compositionto form said catalyst.
 20. The single stage catalytic process of claim19, wherein d ranges from 4.99E-05 to 5.00E-03.
 21. A single stagecatalytic process for direct conversion of ethane to acetic acid bymeans of ethane oxidation comprising the step of oxidizing ethane in areaction mixture comprising ethane and oxygen or a compound capable ofproviding oxygen in a reaction zone in the presence of a catalystcomposition comprising the elements Mo, V, Nb and Pd in the form ofoxides, in the following ratio: Mo_(a)V_(b)Nb_(c)Pd_(d)O_(x) a is 1 to5; b is 0 to 0.5; c is 0.01 to 0.5; d is 4.99E-05 to 5.00E-03; andwherein x is a number determined by the valence requirements of theother elements in the catalyst composition.